x11-drivers/xf86-video-ati: Rework the old/new version dance

While here, introduce a new internal flag, "FORCE_KMS_ONLY_DDX". This
one is used on the build cluster to force the 7.x.y KMS-only version,
even when building on a non-supported kernel (as it's the case on the
build cluster).

PR:		192382


git-svn-id: https://trillian.chruetertee.ch/svn/ports/trunk@1507 058c260c-8361-11dd-a0ac-aa2bafec7d09
Add comment what the patch does.


git-svn-id: https://trillian.chruetertee.ch/svn/ports/trunk@1508 058c260c-8361-11dd-a0ac-aa2bafec7d09